1. Field of the Invention
The present invention relates to a tuning circuit of a Gm (transconductance)-C (capacitance) filter, and more particularly, to a digital tuning circuit of a Gm-C filter for tuning a transconductance using direct current (DC) incorporating variations of a capacitance, instead of using a clock signal, in a Gm tuning mode, while using the clock signal in a capacitance tuning mode.
2. Description of the Related Art
In general, cut-off frequencies of integrated circuit filters are varied by processes and temperature variations. In order to correct the cut-off frequencies, there is required a tuning circuit for performing adaptive correction for compensating for the temperature variations. For example, for a Gm-C filter, there is a need for keeping a constant ratio of the transconductance to the capacitance. Similarly, for an active RC, there is a need for keeping a constant ratio of the resistance to the capacitance.
In addition, the Gm-C filter also requires a tuning circuit for performing the adaptive correction for compensating for the temperature variations. Herebelow, such a tuning circuit of the Gm-C filter will be described with reference to FIG. 1.
FIG. 1 is a block diagram of a typical tuning circuit of a Gm-C filter.
Referring to FIG. 1, the typical tuning circuit of the Gm-C filter includes a voltage controlled oscillator 10, a phase detector 20 and a low pass filter 30. The voltage controlled oscillator 10 includes a Gm cell to generate a predetermined frequency. The phase detector 20 detects phase difference between an oscillation frequency output from the voltage controlled oscillator 10 and a reference frequency. The low pass filter 30 passes only low frequency components of the phase difference signal received from the phase detector 20 to remove noise before outputting the resultant phase difference signal to the voltage controlled oscillator 10.
Here, the voltage controlled oscillator 10 corrects the transconductance according to the phase difference signal received from the low pass filter 30.
As described above, the typical tuning circuit of the Gm-C filter shown in FIG. 1 is a PLL type circuit utilizing the voltage controlled oscillator. That is, the typical tuning circuit of the Gm-C filter compares the oscillation frequency generated by the voltage controlled oscillator and the reference frequency to keep a constant ratio of the transconductance to the capacitance.
The voltage controlled oscillator may be a ring type voltage controlled oscillator, a voltage controlled oscillator using a negative transconductance, or the like.
However, in such a typical tuning circuit of the Gm-C filter as shown in FIG. 1, the voltage controlled oscillator often operates in a large signal until the signal is restricted. Therefore, it may have characteristics different from those of filters operating in a small signal. In addition, the transconductance generated using the voltage controlled oscillator may be different from that generated using a filter. These may become more severe with environmental variations.
Furthermore, because such a correction circuit utilizes a clock signal as a reference signal to correct the frequency characteristics, signals may be affected by the clock signal.